The invention relates to a method for checking the operational capability of a fuel tank ventilation valve of a motor vehicle with an internal combustion engine having an intake manifold pressure-based detection of filling levels.
A computer program and a computer program product, which are suitable for carrying out the method, are the subject matter of the present invention.
Present day internal combustion engines have fuel tank ventilation systems, wherein fuel evaporating in the tank is stored in an activated charcoal filter, which is connected to the intake manifold of the internal combustion engine via a blockable fuel tank ventilation valve. When the fuel tank ventilation valve is open, air is drawn in via a connection of the activated charcoal filter to the atmosphere. This intake air carries the intermediately stored fuel along and supplies it for combustion. The gas quantity drawn in is controlled by the fuel tank ventilation valve such that on the one hand the activated charcoal filter is sufficiently flushed with air and on the other hand no intolerably large disturbances of the air/fuel ratio of the mixture supplied to the internal combustion engine occur.
For the sake of complying with government regulations, a defective fuel tank ventilation valve installed in a fuel tank ventilation system must now be recognized as defective by suitable diagnoses. It is already known how to open a fuel tank ventilation valve during the operation of the engine and how to evaluate a reaction from an air/fuel ratio control loop for diagnostic purposes. The fuel vapor mixed with air from the fuel tank ventilation (regeneration gas) causes a disturbance in the control loop so that the occurrence of the disturbance indicates an operable fuel tank ventilation and thereby in particular an operable fuel tank ventilation valve. This is, for example, described in the German patent application DE 100 43 071 A1.
Provision can thereby be made for the fuel tank ventilation valve to repeatedly open and for a statistical evaluation of the change in mixture, which arises by controlling the valve to open and is detected by means of a lambda probe, to be used for diagnostic purposes. This test can be carried out at idle or when the engine is partially loaded. In so doing, the fuel tank ventilation valve is slowly controlled to open in a ramp-like manner without having the total system take this controlled opening into account. In other words, the air/fuel fraction, which is supplied to the engine via the fuel tank ventilation valve, is not taken into account when said fuel tank ventilation valve is being controlled to open. An intact, respectively defective, fuel tank ventilation valve can be suggested by means of the reaction of the total system to this disturbance variable, which now occurs and represents a deviation in the mixture. If now, however, it is the case that the mass flow directed across the fuel tank ventilation valve does not contain any hydrocarbon molecules, i.e. when “flushing” an unloaded activated charcoal filter, a reaction of the lambda control does not thereby result after opening the fuel tank ventilation valve, and therefore it cannot be determined whether a defective fuel tank ventilation valve is present. In other words, the case can occur that the fuel tank ventilation valve is in fact properly controlled to open; however, a deviation in the mixture is not determined so that—as previously mentioned—a conclusion regarding the operational capability of the fuel tank ventilation valve cannot be made.
The detection of filling levels results for example with the aid of a hot film air-mass meter. Detections of filling levels are also known using an intake manifold pressure sensor. In this kind of detection of filling levels, the gas additionally introduced into the system and thus into the intake manifold is directly measured using an intake manifold pressure sensor.